Tetrapylon

A tetrapylon (Greek: Τετράπυλον, "four gates"), plural Tetrapyla, known in Latin as a "quadrifrons" (literally "four fronts"), is a type of ancient Roman monument of cubic shape, with a gate on each of the four sides: generally it was built on a crossroads.

It was a type of monument common to the architectural vernacular of classical antiquity. The defining quality of this form is the concept of four gates, with four pillars or other supporting structures placed at the corners marking the divisions between them. A tetrapylon could take the form of a single building or multiple, separate structures. They were built as landmarks at significant crossroads or geographical 'focal points', as a 'sub-type' of the Roman triumphal arch, or simply as decorative and aesthetically pleasing ornamental architecture. As applied to a triumphal arch (e.g., the Mausoleum of the Julii at Glanum, Arch of Janus, Rutupiae), a tetrapylon was effectively a 'doubling' of the original form; with a total of four major arched openings, one on each side of the structure (one pair of openings opposite each other along one axis, and a second pair of openings of equal or lesser prominence perpendicular to the first pair; hence a structure with two barrel vaulted passageways, in the form of a cross).

Perhaps the most striking construction at Palmyra in Syria, the Tetrapylon marks the second pivot in the route of the colonnaded street. It consists of a square platform bearing at each corner a tight grouping of four columns. Each of the four groups of pillars supports 150,000kg of solid cornice.

A tetrakionion (τετρακιόνιον), plural Tetrakionia, is a type of tetrapylon in which the central crossing is not roofed, and the four corner-markers exist as four separate structures (i.e.: unconnected overhead).